The present invention relates to a communications network, and in particular to a broadband network used to carry a circuit-connected service such as telephony.
In a conventional telephony network, a local exchange communicates call set-up information to a trunk exchange via a common channel signalling network. Similarly, trunk exchanges communicate via the common channel signaling network. An exchange is pre-configured to access a certain number of circuits in a fixed bearer network. Provided that one of these circuits is free, the exchange captures a circuit and uses the corresponding circuit identification code (CIC) on the common channel signalling network to associate call set-up information with the captured circuit resource in the fixed bearer network.
Increasingly, services such as voice telephony are being carried not over dedicated telephony networks, but over broadband networks designed to support a range of different services including, typically IP (Internet Protocol) data services. Typically, a number of permanent virtual circuits (PVCs) are pre-allocated to act as bearer channels for circuit-connected services such as telephony. Alternatively, if bandwidth on the broadband network is relatively scarce, then instead of the pre-allocated bearing circuits, switched virtual circuits (SVC) are established on a per-call basis as required.
The paper “An efficient reservation connection control protocol for gigabit networks”, Varbarigos et al., Computer Networks and ISDN systems 30 (1998) 1135-1156 describes an efficient reservation virtual circuit (ERVC) protocol in which a start time field in a SETUP packet is set by the source node to the known round trip time for transmission to and from the destination node. Each intermediate node increments that field with a value for the delay on the incoming link from its preceding node, and reserves the capacity requested by the SETUP packet at a time offset from the receipt of the SETUP packet by the respective new value of that field. The destination node returns an ACK packet containing the final value of that field, and the source node delays for an amount equal to the difference between this final value of the field and its original value, and then sends a COMMIT packet followed by data packets. In this way, the COMMIT packet arrives at the final intermediate node exactly at the starting time of the reservation on the outgoing link to the destination node. Thus, maximum efficiency of reservation is achieved for this final intermediate node, and with progressively lesser values of efficiency for the preceding intermediate nodes, but always better efficiency than if the respectively reservations had been made upon transmission by the source node and receipt of the SETUP packet by the intermediate nodes.